This application is based on and claims priority on Japanese patent application 2000-386449, filed on Dec. 20, 2000, the whole contents of which are incorporated herein by reference.
A) Field of the Invention
The present invention relates to a method of forming an oxide film such as alumina (aluminum oxide) and a method of manufacturing a magnetic tunneling junction element by using the oxide film forming method.
B) Description of the Related Art
Magnetic tunneling junction elements are known as megnetoresistive elements to be used for magnetic heads, magnetic memories, magnetic sensors and the like. As a magnetic tunneling junction element manufacture method, a method as illustrated in FIGS. 10 to 12 is known (e.g., JP-A-2000-91668).
In the process illustrated in FIG. 10, on the surface of a ferromagnetic layer 1 made of Fe or the like, an aluminum film 2 of 2 nm in thickness is formed by sputtering. Next, pure oxygen is introduced into a sputtering chamber, and the aluminum layer 2 is oxidized for 10 minutes by setting an oxygen pressure in a range from 20 mTorr to 200 Torr. An alumina film 3 is therefore formed on the surface of the aluminum film 2 as shown in FIG. 11. This alumina film 3 is used as a tunneling barrier film. Thereafter, in the process illustrated in FIG. 12, a ferromagnetic layer 4 made of Coxe2x80x94Fe alloy or the like is formed on the alumina film 3 by sputtering.
As a method of forming an alumina film as a tunneling barrier film, other methods are also known, including (a) a method of exposing an aluminum film in the air to make it subject to natural or native oxidation and (b) a method of subjecting an aluminum film to a plasma oxidation process (for the method (a), refer to JP-2000-91668, and for the method (b), refer to JP-2000-36628).
With the conventional method (b), oxidation becomes likely to be too excessive so that an underlying ferromagnetic layer may be oxidized at the interface with the tunneling barrier film and the variation in a magnetic tunneling resistance may become lower.
With the conventional method (a), it takes a long time, several hours, to complete the oxidation process so that the tunneling barrier film may be formed with pin holes or contaminated by the presence of dusts in the air and the film quality may be degraded.
Although the method illustrated in FIGS. 10 to 12 is an improved version of the method (a), an aluminum film 2 not oxidized is likely to remain under the aluminum film 3 as shown in FIG. 11. The remaining aluminum film 2 lowers the variation in the magnetic tunneling resistance. If oxidation of the aluminum film is insufficient, the electrostatic breakdown voltage of the magnetic tunneling junction lowers and the time-dependent change in the variation in the magnetic tunneling resistance becomes large when the magnetic tunneling junction element is placed in a high temperature environment. For these reasons, the reliability of a magnetic tunneling junction element lowers.
It is an object of the present invention to provide a novel method of manufacturing an oxide film capable of forming a thin oxide film in a short time without oxidizing an underlying layer.
It is another object of the invention to provide a novel method of manufacturing a magnetic tunneling junction element capable of improving the variation in a magnetic tunneling resistance, the electrostatic breakdown voltage, reliability and productivity of the tunneling junction element.
According to one aspect of the present invention, there is provided a method of forming an oxide film comprising the steps of: forming a conductive film by depositing conductive material on an underlying layer capable of being oxidized; and oxidizing the conductive film while oxide of the conductive material is deposited on the conductive film by reactive sputtering in an oxidizing atmosphere, to form a composite oxide film on the underlying layer, the composite oxide film including a first oxide film which is the oxidized conductive film and a second oxide film made of the deposited oxide.
According to this method of forming the oxide film, after a conductive film is formed by depositing conductive material on an underlying layer capable of being oxidized, the conductive film is oxidized while oxide of the conductive material is deposited on the conductive film by reactive sputtering in an oxidizing atmosphere. Accordingly, oxidation of the underlying layer can be prevented because of the presence of the conductive film, and oxidation of the conductive film is suppressed more as the oxide of the conductive material deposited becomes thicker. Namely, the conductive film is not positively subjected to the oxidation process, but the phenomenon is utilized by which phenomenon the conductive film is oxidized while the oxide of the conductive material is deposited on the conductive film by reactive sputtering in an oxidizing atmosphere. Therefore, by optimizing a ratio of the thickness of the conductive film to the thickness of the second oxide film, oxidation can be stopped just at the interface between the underlying layer and conductive film. For example, by setting the thickness of the second oxide film to the thickness required for completely oxidizing the conductive film, it is possible to oxidize only the conductive film and prevent the underlying layer from being oxidized.
According to this method of forming the oxide film, during the reactive sputtering in the oxidizing atmosphere, oxygen plasma reacts also with the conductive film so that the conductive film can be oxidized sufficiently even at a low temperature. The composite oxide film of a good quality can be formed which has the first oxide film of the oxidized conductive film and the second oxide film of the deposited oxide. Since the sputtering process is used, a thin oxide film can be formed in a short time.
In the method of forming a oxide film, as the conductive material, metal such as aluminum, titanium and magnesium or semiconductor such as silicon can be used.
According to another aspect of the present invention, there is provided a method of manufacturing a magnetic tunneling junction element comprising the steps of: forming a conductive film by depositing conductive material on a first ferromagnetic layer capable of being oxidized; oxidizing the conductive film while oxide of the conductive material is deposited on the conductive film by reactive sputtering in an oxidizing atmosphere, to form as a tunneling barrier film a composite oxide film on the first ferromagnetic layer, the composite oxide film including a first oxide film which is the oxidized conductive film and a second oxide film made of the deposited oxide; and forming a second ferromagnetic layer on the tunneling barrier film, the second ferromagnetic layer facing the first ferromagnetic layer.
With this method of manufacturing a magnetic tunneling junction element, the tunneling barrier film of the magnetic tunneling junction element is formed by employing the above-described method of forming an oxide film. Since the tunneling barrier film made of the first and second oxide films can be formed by oxidizing only the conductive film without oxidizing the first ferromagnetic layer, variation in the magnetic tunneling resistance can be improved. Since an oxide film of a good quality to be used as the tunneling barrier layer can be formed in a short time, the electrostatic breakdown voltage, reliability and productivity of magnetic tunneling junction elements can be improved.
As above, an oxide film of a good quality can be formed on the underlying layer in a short time without oxidizing the underlying layer.
The oxide film to be used as the tunneling barrier film formed by the method of forming an oxide film improves the variation in magnetic tunneling resistance, electrostatic breakdown voltage, reliability and productivity.